Hot melt adhesives (HMA's) are solid at room temperature and are generally applied to a substrate while in a molten state. Hot melt adhesives can be characterized into two types, hot melt pressure sensitive adhesives (HMPSA's) which are tacky to the touch even after the adhesive has solidified and non-tacky hot melt adhesives which will be denoted as simply HMA's. HMA's have been packaged in a variety of forms ranging from small pellets which can be vacuum fed into a melting device, to drum sized quantities. The packaging of HMA's has typically been unproblematic, since after the adhesive is solidified, it will maintain its shape and not adhere to the container, other pieces of hot melt, the operator's hands, machinery, etc. until remelted. Underwater pelletizing devices as well as stripforming or rotoforming HMA's onto a cooled belt have been used extensively for packaging these types of products.
HMPSA's on the other hand, present a variety of challenges. Historically, these adhesives were, and continue to be, provided in the form of blocks. Due to the tacky nature of these materials, there are problems associated with handling and packaging them. The solid HMPSA blocks not only stick to each other and adhere to the operator's hands and mechanical handling devices, but they also pick up dirt and other contaminants. Additionally, certain high tack formulations result in blocks that will deform or cold flow unless supported during shipment. The needs and advantages for providing tackless or nonblocking hot melt pressure sensitive adhesive forms and packages are apparent and various ways of accomplishing this have been developed.
EP 0115307, published Jan. 1, 1984, relates to contacting a stream of molten adhesive with a polymeric dispersion. The examples found on p. 10 and 11 utilize about 40% by weight and about 20% by weight of the parting agent in water. This process is used for "reducing the tackiness of a hot melt adhesive in slat form". See p. 3 lines 17-18.
In German patent 22 48 046, the hot melt adhesive is squeeze-cut into pillow-shaped pieces, the pieces subsequently cooled and solidified.
U.S. Pat. No. 3,723,035 to Franke teaches an apparatus for an improved method of packaging hot melt adhesives. This machine produces what is known in the art as "pillows." For pressure sensitive adhesives that are manufactured with this method, it is known to coextrude a non-tacky coating on the exterior of the pillow to prevent the individual pillow-shaped pieces from subsequently blocking together. The coextrusion coating may comprise a single component such as polyethylene polymer, or polymer, tackifier and wax. This coating then prevents the "pillows" from blocking together into a solid mass at room temperature.
WO 96/00747 published Jan. 11, 1996 teaches a process for coating hot melt adhesives to provide a tackless surface. In this method, molten film is sprayed onto the surface of an extruded adhesive. The surface of the coated adhesive is then heated to a temperature for a period of time sufficient to remelt the film forming polymer so as to form a continuous coating thereof but which process is insufficient to appreciably melt the adhesive. The example teaches a coating of approximately 3 mils (0.08 cm) applied to the hot melt adhesive.
Other references pertain to certain hot melt compositions in pellet form and/or methods of pelletizing hot melt adhesives.
U.S. Pat. No. 5,057,571 to Malcolm et al. discloses, at column 8 line 59 to column 9 line 11, a method for pelletizing a preblend of a radial block copolymer and a tackifying resin. This preblend is then used to make hot melt adhesives for disposable article construction. The preblend is manufactured on an extruder and pelletized and then subsequently added to a conventional hot melt mixer and combined with additional ingredients to make a finished adhesive composition. "The handling of the preblend pellets can be improved by treating the surface of the pellets with a nontacky inorganic coating. Such coating can comprise aqueous dispersions of water insoluble waxes, fatty acid esters, and other known anti-blocking agents. Useful anti-blocking agents can include such materials as silica, talc, gypsum, calcium oxide, magnesium oxide, etc." (Column 9, lines 1-8).
U.S. Pat. Nos. 4,645,537 issued Feb. 24, 1987; 4,576,835 issued Mar. 18, 1986 and 4,774,138 issued Sep. 27, 1988 all to Gardenier et al. teach aqueous release agents for temporary antiadhesive surface-finishing of tacky hot melt adhesives to be processed into pellets, granulates, cubes, flakes and the like. No specific hot melt compositions are taught.
EP 0 410 914 A1 published Jan. 30, 1991 teaches a two step process of making nontacky plastic particles by first extruding molten plastic material into contact with a cooling fluid containing a non-sticky material that is compatible with the plastic material, cutting the plastic material into particles, separating the particles from the cooling fluid and then contacting these particles with a second compatible non-sticky material. At column 6, lines 45-49, it is stated, "Examples of suitable non-sticky materials include powders, silicones, and surfactants. The preferred non-sticky materials are powdered polyolefins. More preferred powders are powdered polyolefin waxes."
Collectively, the art teaches pelletizing adhesives using methods which employ waxes, silicones or surfactants as pelletizing aids to detackify the surface. These types of materials are low in molecular weight. Additionally, the majority of such ingredients, with the exception of some waxes, tend to comprise polar constituents. Accordingly, waxes, silicones and surfactants typically bloom to the surface causing detackification of the pressure sensitive adhesive, unless employed at only very low concentrations.
Further, various pelletizing aids and methods of pelletization have been taught in the art for various polymers and elastomers.
For example, U.S. Pat. No. 4,359,492, issued to Schlademan Nov. 16, 1982 teaches that thermoplastic elastomers useful in formulating certain pressure sensitive hot melt adhesives are normally very tacky and thus are hard to handle. Thus, the patent relates to a method of eliminating problems due to this tackiness by dusting the elastomer pellets with a friable polymer resin having a ring and ball softening point of at least 95.degree. C. Exemplified is a tacky styrene-isoprene block copolymer consisting of 15% by weight styrene and 85% by weight isoprene extruded into strand having a diameter of about 1/8" (about 3 mm). The strands were coated by being dusted with a powder made by pulverizing a poly-alpha-methylstyrene resin having a ring and ball softening point of 141.degree. C. The dusted strands contained less than 2% by weight of the resin and were chopped in a paper cutter to form free-flowing pellets. "Similar, but slightly less effective, results were obtained by dusting the strands of tacky polymer with powdered polyterpene resin having a softening point of about 95.degree. C."
However, Comparative Example 3, of U.S. Pat. No. 5,322,731 issued to Callahan Jr. et al., Jun. 21, 1994; describes the formation of adhesive beads comprising pressure sensitive adhesive cores surrounded by an essentially discontinuous organic rosin ester powder coating. A dispersion of filtered adhesive cores and Foral 85 55 WK at a 4% loading, dry weight basis with regard to the adhesive cores was agitated at about 400 rpm and 65.degree. C. for two hours. The resulting beads were filtered and dried with constant agitation. The resulting beads agglomerated and were not free-flowing.
Since pressure sensitive adhesives are typically considerably tackier and more compliant than neat polymers, the pelletizing aids and methods of pelletization that have been employed successfully in the polymer field have found limited utility in the field of pressure sensitive hot melt adhesives. Accordingly, there has been a long felt need for pelletized hot melt pressure sensitive adhesives and methods of producing such pellets.